The present invention relates to a pressure sensor for measuring pressure, and particularly to a pressure sensor for measuring, by using a thermal pressure detecting element, heat quantities deprived by means of a pressure receiving diaphragm which is disposed as to oppose a heating element included in the thermal pressure detecting element or a portion heated by the heating element with being remote therefrom by a specified distance.
A pressure sensor wherein a deflection value of a diaphragm is measured based on a substantially non-ambiguous functional relationship that is satisfied between a pressure of a measuring fluid and a deflection value of the diaphragm formed on a cylindrical body which receives pressure of the fluid with the use of a strain gage formed onto the diaphragm through film forming techniques or photolithographic techniques for obtaining the pressure of the fluid proportional to the deflection value of this diaphragm is widely applied for detecting an amount of absorbed air of an internal combustion engine or for detecting a hydraulic pressure of a brake of a vehicle.
FIG. 22 is a sectional view of a conventional pressure sensor as disclosed, for instance, in Japanese Unexamined Utility Model Publication No. 137242/1986 (microfilm of Japanese Utility Model Application No. 19572/1985).
In FIG. 22, numeral 101 denotes a metallic cylindrical body, numeral 102 a semiconductor single crystalline plate including a strain gage 103, wherein the semiconductor single crystalline plate comprises, for instance, a silicon substrate. In the arrangement of FIG. 22 wherein the semiconductor single crystalline plate 102 is adhered to the metallic cylindrical body 101, the difference in materials between the metallic cylindrical body 101 and the semiconductor single crystalline plate 102 resulted in the fact that strain was apt to occur in the semiconductor single crystalline plate 102 forming the diaphragm owing to difference in coefficients of linear expansion at the time of temperature variations, thereby causing measuring errors. Further, since a pressure of the measuring fluid is directly applied to the semiconductor single crystalline plate 102, a sufficiently strong bonding strength was required between the metallic cylindrical body 101 and the semiconductor single crystalline plate 102.
In FIG. 23, numeral 104 denotes a metallic cylindrical body comprising, for instance, a cut pipe material which might be a stainless steel pipe. A metallic thin film 105 welded to the cylindrical body 104 is formed of a thin plate of rolled material, and owing to the fact that this is a rolled material, it is formed to assume a uniform film thickness as well as a smooth surface. The metallic thin film 105 is formed of a material that is identical to that of the cylindrical body 104. A silicon oxide thin film 106 that functions as an insulating film is formed on an upper surface of the metallic thin film 105. Plasma CVD methods are employed for forming the silicon oxide thin film 106. Then, a silicon thin film forming a strain gage 107 is formed onto the silicon oxide thin film through plasma CVD methods. Etching of the silicon thin film is performed to remove portions other than partial portions of the silicon thin film as shown in FIG. 23, and the strain gage 107 is formed by the remaining silicon thin film. Further, an circuit might be arranged by forming an electrode by performing vapor deposition of metal such as gold on to the strain gage 107, attaching a lead wire to the electrode by means of ultrasonic bonding, and suitably connecting the electrode and the lead wire.
The conventional pressure sensor as shown in FIG. 22 and FIG. 23 is a pressure sensor employing a strain gage wherein the diaphragm is strained through pressure of measuring fluid applied to the diaphragm and the strain is measured by the strain gage on the diaphragm. There is also employed a pressure sensor for detecting a deflection of a diaphragm as a change in capacity.
FIG. 24 is a sectional view (a) and top views (b), (c) of a conventional pressure sensor of capacity detecting type as disclosed, for instance, in Japanese Unexamined Patent Publication No. 56233/1985.
In the drawings, numeral 108 denotes a base having an electrode 109 in a central portion on its upper surface and a correction electrode 110 at its peripheral edge portion, both in a concentric manner, while a through hole 111 is formed in a clearance formed between these electrodes. Numeral 112 denotes a diaphragm having an electrode 113 on its surface as to oppose the electrode 109. Numeral 114 denotes glass beads for gap adjustment interposed between the substrate 108 and the diaphragm 112 for forming a gap 115 between the electrodes 109, 113. The pressure sensor is so arranged that the gap 115 in the central portion becomes smaller when pressure P is applied onto the diaphragm 112 whereby capacitance between the electrodes 109, 113 is increased. By utilizing a substantially non-ambiguous functional relationship that is satisfied between the change in capacity and the pressure of measuring fluid, it is aimed to measure the pressure.
Due to the above arrangement of the conventional pressure sensor, when using the strain gage formed on the silicon substrate, no satisfactory bonding strength can be secured between the cylindrical body and the silicon substrate on which the strain gage is formed, so that the pressure of the measuring fluid cannot be applied directly onto the silicon substrate to measure the pressure of the measuring fluid. Therefore, it was necessary that the pressure act onto a buffering agent in a different chamber by using the diaphragm that is deformed by the measuring fluid whereupon the pressure of the buffering agent is measured by using the strain gage on the silicon substrate.
When using the strain gage of silicon thin film formed on the metallic diaphragm, it was not easy to directly form the strain gage of silicon thin film on the metallic diaphragm for receiving pressure through mass production in a lump sum since devices for silicon substrates (for silicon processing) could not be concurrently used.
Moreover, in a pressure sensor of capacity detecting type, an insulating layer needs to be formed on the metallic diaphragm and an electrode for capacity detection need to be formed thereafter through photolithographic techniques or the like. In this manner, when using a metallic diaphragm, the metallic diaphragm needs to undergo film forming or photolithographic processes so that film forming devices or photolithographic devices that are suitably used for silicon substrates cannot be used. Further, using a silicon substrate resulted in a complicated structure of the pressure sensor so that a drawback was presented that no pressure sensor of low cost and high reliability could be manufactured.
It is an object of the present invention to realize a simple thermal pressure sensor for solving the conventional problems. A thermal pressure sensor is arranged to thermally detect a displacement value of a diaphragm which receives pressure, wherein variations in heat quantities that are deprived in accordance with displacements of the diaphragm are obtained from a heating element of a detecting elements or a portion heated by the heating element which is remote from the diaphragm by a specified distance.
It is an object of the present invention to obtain a pressure sensor of high reliability and of low cost wherein measuring elements can be formed onto a silicon substrate through mass production in a lump sum by using conventional manufacturing techniques and devices which are applied to silicon substrates, wherein no processing of the metallic diaphragm is required while utilizing a metallic diaphragm formed on a cylindrical body as a pressure receiving body, and wherein no additional chamber for holding buffering agents is required since no external force is directly executed on the measuring element at the time of measuring pressure.
The pressure sensor of the present invention is arranged to be a pressure sensor comprising a diaphragm arrangement having a first surface which receives pressure and a thermal detecting portion disposed as to oppose a second surface of the diaphragm arrangement, wherein displacement values of the diaphragm owing to variations in pressure are thermally detected at the thermal detecting portion. With this arrangement, a surface of the diaphragm which receives pressure does not need to undergo film forming or photolithographic processes whereby main portions of thermal pressure detecting elements might be formed onto a silicon substrate by large quantities in a lump sum through simple manufacturing processes so that it is possible to improve accuracy and reliability of the thermal pressure detecting elements and to obtain a pressure sensor of low cost.
Since pressure is measured in a thermally non-contacting manner, external force is not directly executed on the thermal pressure detecting elements during measurement so that no bonding strength which endures pressure of measuring fluid needs to be maintained between the cylindrical body and the thermal pressure detecting elements, and it is enabled to achieve an easy arrangement and to obtain a pressure sensor of low cost.
Further, the pressure sensor is so arranged that the thermal detecting portion includes a heating means, and displacement values of the diaphragm are measured as amounts of variations in resistance values of the thermal detecting portion. With this arrangement, the heating portion itself is provided with a function of detecting temperature, and owing to the fact that the measuring circuit for measuring resistance values of the heating portion is simple, it is possible to obtain a pressure sensor of low cost.
Further, the thermal detecting portion includes a heating means which performs heating to a specified temperature, and displacement values of the diaphragm are measured as amounts of variations in current values of the thermal detecting portion. With this arrangement, the heating portion itself is provided with a function of detecting temperature, and by arranging a bridge circuit, current values for measurement might be accurately controlled so that it is possible to improve sensitivity of the thermal pressure detecting elements and to obtain a pressure sensor of low cost.
The thermal detecting portion disposed as to oppose the diaphragm is arranged to be smaller in size than the diaphragm. With this arrangement, the thermal detecting portion can be arranged to oppose a portion of the diaphragm which exhibits large displacements, and it is possible to obtain a pressure sensor of high sensitivity wherein amounts of variations in resistance, voltage and current due to pressure are large. Further, since the thermal detecting portion is small, it is possible to obtain a pressure sensor which power consumption is reduced.
Further, the thermal detecting potion includes a heating means, and further includes a temperature detecting portion adjacent to the heating means, and displacement values of the diaphragm are measured as amounts of variations in temperature of the temperature detecting portion. With this arrangement, it is possible to employ a detector of high sensitivity as the temperature-detecting portion and to obtain a pressure sensor of favorable sensitivity.
By further including a temperature compensating means for measuring and compensating an ambient temperature, it is possible to eliminate influences owing to variations in environmental temperature during usage by measuring the ambient temperature, and it is possible to obtain a pressure sensor of high reliability.
By further including a second temperature compensating means for measuring and compensating a temperature of the diaphragm, it is possible to eliminate influences owing to variations in environmental temperature during usage and in the temperature of the diaphragm by measuring the ambient temperature and the temperature of the diaphragm, and it is possible to obtain a pressure sensor of high reliability.
The arrangement further includes a second thermal detecting portion disposed as to oppose an end portion of the second surface of the diaphragm, that is, a second thermal detecting portion is disosed at outside portion of the diaphragm which is not displaced owing to pressure to achieve a reference output which is not varied owing to pressure. With this arrangement, it is possible to eliminate in-phase noise components by obtaining differences between the reference outputs and signal outputs, and it is possible to obtain a pressure sensor of low cost and of high reliability. Further, the provision of the second thermal detecting potion enables it to measure a temperature of the diaphragm to eliminate influences owing to variations in environmental temperature during usage and in temperature of the diaphragm, and it is possible to obtain a pressure sensor of high reliability.
By arranging a bridge circuit by means of the first thermal detecting portion and the second thermal detecting portion, it is made possible to perform signal processing through a simple circuit, and it is possible to obtain a pressure sensor of high reliability.
The pressure sensor according to the present invention is arranged to be a pressure sensor comprising a diaphragm arrangement having a first surface receiving pressure with a thermal detecting portion being formed in a concave portion of a substrate such that a diaphragm and the thermal detecting portion are disposed as to oppose each other through the concave portion of the substrate, and a thermal detecting portion disposed as to oppose a second surface of the diaphragm arrangement, wherein displacement values in the diaphragm owing to changes in pressure are thermally detected by the thermal detecting portion. With this arrangement, a distance between the diaphragm which serves as a pressure receiving surface and a thermal detecting element can be accurately controlled, and it is possible to obtain a pressure sensor of high reliability.
Further, spacers for regulating an opposing distance between the thermal detecting portion and the diaphragm are provided on the substrate on which the thermal detector portion is formed. With this is arrangement, spaces can be formed on a substrate in large quantities in a lump sum by using photolithographic techniques, and the distance between the diaphragm and the thermal detecting element can be accurately controlled so that it is possible to obtain a pressure sensor of high reliability.
The pressure sensor according to the present invention is so arranged that a second diaphragm arrangement which is disposed as to oppose the second surface of the first diaphragm and at least a part of which is supported on a silicon substrate is formed, and that a thermal detecting portion is arranged on the second diaphragm. With this arrangement, a diaphragm formed of heat-insulating supporting film of which strength is sufficiently maintained can be easily manufactured, and heat-insulating characteristics can be improved so that it is possible to obtain a pressure sensor of low power consumption and of superior sensitivity.
By arranging the heat-insulating supporting film which supports a heating portion as a bridge like arrangement, it is possible to obtain an arrangement of even more superior heat-insulating characteristics, of low power consumption, and of improved sensitivity, and owing to the bridge arrangement, no displacements occur in a thermal pressure detecting element portion since pressure acting on both ends thereof are identical, and it is possible to obtain a pressure sensor having outputs of favorable linear formation characteristics.
A void is formed on a side of the second surface of the silicon substrate, and a length of the void from the second diaphragm surface in the normal direction is set to be larger than a distance between the second diaphragm and the diaphragm receiving the pressure, whereby heat flow components flowing in a direction of the void can be restricted to thereby decrease the amount of consumed electric power.
Since the pressure sensor according to the present invention is further provided with a protecting portion covering the thermal detecting portion, reliability thereof can be further improved, and since a pressure of an interior of the protecting portion is set to be at atmospheric pressure or higher than atmospheric pressure, the sensitivity of the pressure detecting element can be continuously maintained to be constant and heat from the heating portion can be efficiently transmitted to the diaphram, and it is possible to obtain a pressure sensor of high reliability.